1. Field of the Invention
The present invention relates generally to turbomachinery and, more particularly, to a method of, and apparatus for, both passively and actively reducing the noise energy caused by turbomachinery.
More specifically, a primary purpose of this invention is to reduce the blade rate frequency (BRF) tones which radiate from turbomachinery. The acoustic radiation can be an irritant to personnel and may excite other adjacent structure into vibration. By minimizing the net unsteady pressure field on the blades, the level of radiated sound can be reduced. The approach of this invention is to create an unsteady pressure field that is out of phase with the existing unsteady pressure field acting on the blades. A variable tip clearance around the periphery of the rotor is used to create this unsteady pressure field. Additionally, the variable tip clearance shroud can be rotated with respect to a stationary frame to obtain the necessary phase relation for tone reduction. The invention is conducive to adaptive control methods, where the shroud position is adjusted until an acoustic signal is minimized.
2. Discussion of the Prior Art
The principal noise sources of an axial-flow turbomachine can be attributed to the viscous flow over the airfoils or blades. The acoustic spectrum is characterized by broadband radiation and discrete frequency tones occurring at integer multiples of the BRF. Broadband noise can be attributed to the shedding of vorticity from the blade trailing edges and from pressure fluctuations in regions of unsteady and turbulent flow. Discrete frequency noise is due both to the steady and unsteady blade loading.
Several design expedients are currently used to achieve some tonal reduction, such as blade-row vane number, blade-row spacing, skew distribution, shaft speed and isolation. For a given design, BRF noise can be accomplished by reducing the nonuniformities in the flow which are the principal cause of unsteady blade loads and subsequent discrete frequency noise. By eliminating flow obstructions or placing them far from the turbomachine, flow nonuniformities can be reduced. While these are effective to varying degrees, the discrete tones require further reduction. The various methods of controlling this unsteadiness have been classified as either active or passive.
Active control describes a class of techniques in which external energy is introduced to cancel the existing acoustic sources. A system of speakers and microphones are used to synthesize xe2x80x9canti-noisexe2x80x9d of the appropriate frequency content which minimizes the error signal from the microphone. These systems are difficult to implement physically. Active control also describes methods which generate unsteady fluid dynamic forces to reduce unsteadiness. In many circumstances, robust actuators of sufficient response time are not available.
Passive control denotes methods which take advantage of the unique physics of the particular flow situation to reduce the net acoustic radiation. The muffler on an automobile would be an example of a passive control device. Passive methods are generally less complex and less expensive than active control methods.
The present invention incorporates both active and passive noise control techniques. In its rudimentary form, the technique is passive. Adaptive, yet passive control can be utilized with the addition of a feedback loop which minimizes the measured acoustic radiation at select BRF harmonic tone(s). The benefits of the concept can be further extended by incorporating active techniques with additional mechanical complexity.
Generally typical of the prior art relating to noise suppression in turbomachinery are the following U.S. patents:
U.S. Pat. No. 4,531,362 issued Jul. 30, 1985 to Barry et al.;
U.S. Pat. No. 4,104,002 issued Aug. 1, 1978 to Ehrich;
U.S. Pat. No. 3,832,085 issued Aug. 27, 1974 to DeFauw et al.; and
U.S. Pat. No. 3,286,914 issued Nov. 22, 1966 to Baverstock.
Numerous other patents have been studied by the applicants. While in many instances they appear, at first glance, to have the physical features of the subject invention, in fact, they are really directed to the enhancement of aerodynamic performance, such as efficiency and stall margin.
Of the more pertinent patents, Baverstock utilizes a scroll surrounding a centrifugal fan which is perforated and accordion-shaped in a direction parallel to the rotor axis, such that a cross section shows a saw-tooth or zig-zag pattern. Behind the scroll is sound absorbent material which is backed by the casing. This concept is unique to centrifugal fans and concerns sound absorption, only. It is now widely known that the principal noise sources in a centrifugal fan are at the cutoff of the scroll, a concept not addressed in the patent.
Erich discloses an acoustic duct for a gas turbine engine. In this instance, sound-absorbing linings for the inlet of the engine are placed in a helical pattern to absorb spinning mode acoustical energy. Noise suppression is maximized by aligning the angle of the helix to the direction of the wave front generated by the first stage of the rotor. In several variations of the invention, alternate strips have different widths which accommodate absorption at a number of different frequencies, or acoustically treated strips are alternated with untreated strips. In summary, noise is suppressed in a compressible flow duct by scattering spinning mode acoustic pressure fields through the use of circumferentially spaced and helically extended strips of sound-absorbing material.
In the Barry et al. patent, cavities surrounding the shroud or end-wall of an engine fan are tuned for resonance at the known flutter frequency of the rotor, the concept being aimed at reducing blade flutter and the subsequent high blade stresses and possibility of failure. The cavities are placed around and beneath the circumference of the end-wall and communicate to the duct through openings in the end-wall near the tips of the blades. This method of reducing flutter is superior to earlier known part-span shrouds, or clappers, which add weight and manufacturing cost. The cavities are arranged with axes parallel to the extent of the end-wall. Their length is determined to be one quarter of the wavelength of the known flutter frequency. When the cavities resonate at the appropriate frequency, the pressure-waves at the openings of the tubes are out of phase with the incoming waves, thereby achieving partial cancellation of the incoming pressure waves. The openings of the cavities are staggered within sets of four. The first of the four has an opening near the leading edge of the blade, and the last of the four has an opening near the trailing edge of the blade.
The most relevant of the patented techniques known to the applicants is disclosed in DeFauw et al. In this patent, a shroud surrounding an automotive axial-flow fan is provided with circumferentially spaced projections from the internal surface which are said to reduce the fan-shroud combination noise level. The shroud projections have a finger-like appearance, are oriented parallel to the shroud axis, equally spaced, and tapered on the upstream end. The mechanism of noise reduction is claimed to be the circumferentially undulating pattern to the axial flow of air past the shroud, not the rotor. The number of projections is not mentioned and, therefore is seemingly unimportant. Additionally, the large tip clearance illustrated in the drawing would not be effectual in modifying the unsteady forces on the rotor. In summary, the DeFauw patent utilizes the shroud aerodynamics, rather than an additional source of unsteadiness or noise, to reduce the fan shroud combination broadband noise level.
It was with knowledge of the prior art as just described that the present invention has been conceived and is now reduced to practice.
In accordance with the invention, apparatus is disclosed for reducing acoustic radiation which occurs during the operation of turbomachinery. The apparatus of the invention includes a shroud having a generally cylindrical inner surface and a coaxial rotor having a plurality of blades extending radially outwardly at equally spaced circumferential locations. The inner surface of the shroud is circumferentially contoured such that the tip clearance between each of said blades and the inner surface is caused to vary in a periodic manner upon rotation of said rotor. This creates a new periodic unsteady pressure field which is substantially equal to, and out of phase with, an existing periodic pressure field resulting mainly from nonuniform inflow into said blades and results in the radiation from the turbomachinery of reduced blade rate frequency tone(s). The inner surface of the shroud may have a circumferential sinusoidal contour whose periodicity is an integral multiple of the number of the blades. A plane of the tip end of each blade, or attack plane, is preferably angularly disposed relative to a plane perpendicular to the axis of the shroud, and the contoured inner surface includes a plurality of parallel grooves generally aligned with the attack plane of each of the blades. Additionally, the shroud may be indexed about its longitudinal axis until an optimized reduction of blade rate frequency tones has been achieved. Furthermore, the inner surface of the shroud may be defined as a Fourier series enabling a simultaneous reduction of a plurality of harmonics of the blade rate frequency tones.
In the present invention, a shroud containing equally-spaced projections from its internal surface is used to reduce the fan-shroud combination noise level by creating a circumferentially undulating pattern to the tip clearance. The present invention differs significantly from the prior art in that an additional source of unsteady force is generated on the rotor by the variable tip clearance which is out-of-phase with existing unsteady forces on the fan, thereby reducing the acoustic radiation from the fan at blade passage frequency and multiples concurrently. The level of cancellation is determined by the shroud or end-wall geometry and the proper angular orientation of the end-wall with respect to the inflow disturbances. The technique of the subject invention is inherently self-tuning and lends itself readily to the advantages of passive adaptive control in the event of variations in the fan inflow.
Accordingly, a primary object of the invention is to provide a method of, and apparatus for, both passively and adaptively reducing the noise energy level caused by turbomachinery.
Another object of the invention is to provide a shroud or end-wall for a multi-bladed fan rotor constructed with a contour whereby the clearance between the shroud and the tips of the blades caries with rotation, thereby reducing the discrete frequency tones which are generated. For purposes of the invention, it makes no matter whether the blades rotate relative to the shroud or the shroud rotates relative to the blades.
A further object of the invention is to provide such an expedient which, in one instance, is passive, requiring no external energy source, and in another instance is active, requiring external energy.
Still another object of the invention is to provide a noise abatement technique which is less complicated than other techniques. According to the technique of the invention, a sinusoidally shaped shroud or end-wall replaces an existing smooth end-wall and requires no other alterations.
Yet another object of the invention is to provide reduction of several harmonics of the BRF tones which may be achieved simultaneously by employing an end-wall or shroud contour whose inner surface is defined as a Fourier series.
Additionally, the technique of the invention can be made adaptive yet passive for other inflow conditions by employing an actuator to rotate the end-wall until the acoustic radiation, as determined by a microphone or hydrophone, is minimized. Inflow conditions change to the turbomachine when system losses vary, such as valve closure or the introduction of new system Components such as elbows, obstructions or additional lengths of duct.
Time variations of tip clearance rather than spatial variations of the tip clearance could also be achieved using a method of active control to move the end-wall in the radial direction. In this instance, the periodicity of the end-wall, and hence high harmonic numbers, would not be limited by machining capability.
Other and further features, advantages, and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of this invention, illustrate some of the embodiments of the invention, and, together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.